1. Field of the Invention
The solution according to one or more embodiments of the present invention generally relates to the electronic field. More specifically, this solution relates to the indexing of electronic devices.
2. Discussion of the Related Art
Electronic devices are generally integrated in dice, which are formed in a large number of portions of a wafer. Particularly, in a production process of the stepper shot type, each stage of the production process is not performed concurrently on the whole wafer, but step by step repeating the same operations on different shot areas thereof (by moving a smaller photolithographic mask accordingly).
In this context, it is of the utmost importance to be able to determine the original position of the dice of the electronic devices in the wafer (before their separation). For example, this information is very useful for a quality management of the production process. Indeed, several characteristics of the electronic devices (for example, their functional parameters, performance and reliability) depend significantly on the position of the corresponding dice in the wafer (for example, because of changes in the crystallographic structure of the wafer through its extent). Therefore, when some electronic devices are subject to failures during their operation and are then returned to a corresponding manufacturer, the knowledge of their position in the wafer facilitates the analysis of the failures and makes it possible to devise corresponding improvements in their production process. The same information may also be useful during a test of the electronic devices at the wafer level, known as Electrical Wafer Sorting (EWS). Indeed, in this way it is possible to store the position of any defective electronic devices that did not pass the test, so that they may be identified and discarded after the corresponding dice have been cut (without the need of marking the dice of the defective electronic devices during the test to discriminate them—for example, with ink dots).
For this purpose, it is known in the art to form an index on each die (when it is still included in the wafer), which index indicates the position of the die in the wafer. Particularly, in the case of the production process of the stepper shot type, the index has a composite structure with a shot index (indicating the position of the corresponding shot area in the wafer) and a die index (indicating the position of the die in the corresponding shot area). Typically, each (shot and die) index is defined by a row index and a column index, which define a row coordinate and a column coordinate, respectively, in a corresponding matrix. Particularly, a generic index may be implemented with a ruler (for example, being formed in a surface metallic layer of the die), which ruler defines an ordered alignment of locations (referred to as dots) each one associated with a corresponding number; a marker selects a specific dot (for example, by means of the erasure of a corresponding portion of the metallic layer), and then the corresponding number. In this case, the die indexes and the rulers of the shot indexes of all the dice may be formed during a selected stage of the production process by means of a corresponding mask (which replicates the same structures in the different shot areas); the markers of the shot indexes are instead formed by exploiting an additional mask (designed to form them at the same position in all the corresponding dice), which additional mask is however slightly displaced at every shot so as to form these markers at different positions in every shot area. An example of the above-mentioned indexing technique is described in US-A-2008/0153250.
A drawback of the indexing techniques known in the art is their limitation in the number of dice that can be indexed in the same wafer. Indeed, the dots of the rulers cannot be smaller than a minimum size (for example, 1 μm×1 μm), in order to allow their correct inspection; in addition, the area of the dice that can be used to form each ruler is constrained (for example, with a length of 15 μm). As a result, the maximum value of each row and column index is relatively low (about 15 μm/1 μm=15), with a corresponding limitation in the range of each shot and die index (15×15=225 in the example at issue). This drawback is particularly acute in the die indexes, since the modern production processes easily exceed the above-mentioned number of dice in each shot area; in this case, it is not possible to implement any indexing of the dice (with a detrimental effect on the quality of the corresponding production process).
Different indexing techniques are also known in the art.
For example, in JP-A-10012527 each index is represented with a binary code; the bits of the index are defined in corresponding locations (arranged along a straight line) to the value 1 in presence of a predefined slit or to the value 0 in its absence. The same document also describes other embodiments wherein the index is represented with a number in a base higher than 2; particularly, the digits of the index are represented by the length multiple of a predefined value of corresponding teeth (in this case, with the index that has a variable length), or by the depth multiple of a predefined value of slits at the corresponding locations, with the value 0 that is represented by the absence of any slit (in this case, with the slits that extend transversally to the arrangement of the locations).
Alternatively, in JP-A-61142734 each bit of the index is represented by the length of a corresponding bar (a long bar for the value 0 and a short bar for the value 1).
Moreover, in US-A-2003/127718 each bit of the index is represented at a corresponding location by the presence or the absence of a recess.